Large power generation turbines have a number of uses, such as driving a shaft to a generator or on the propeller of a large ship, for example. These turbines use thermal barrier coatings as blade outer air seal coatings in the hot section of the turbine. Due to the slow response rate of these large turbines, rub does not typically occur early in life like it does in aircraft engines.
When the rub does happen, it is at a slow interaction rate, high temperature and generally with high tip speeds. In this combination of conditions, the metal of the blade tips smears and galls to the ceramic blade outer air seal. This transferred material causes an increase in roughness on both the blade tips and outer air seal with associated aerodynamic looses and creates high spots on the outer air seals that then contact and remove material from, normally not contacting, blade tips resulting in an increased tip gap and a loss in performance.
These large power generation turbines will have better performance if metal transfer can be avoided.